Inter-cellular interference cancellation

ABSTRACT

With respect to a first wireless transmit/receive unit (WTRU), at least one WTRU near the periphery of other cells is determined. Out of the at least one WTRU near the periphery, at least one WTRU nearby that WTRU is determined. The first WTRU codes are provided of the at least one nearby WTRU. A contribution of the at least one nearby WTRU is canceled from a received signal at a first WTRU, producing an interference canceled signal. Data of the first WTRU is detected from the interference canceled signal.

FIELD OF INVENTION

This invention generally relates to wireless communication systems. Inparticular, the invention relates to cross cell interference reductionin such systems.

BACKGROUND

Cross cell interference is a problem in wireless communication systems.FIG. 1 is an illustration of cross cell interference. As illustrated, awireless transmit/receive unit (WTRU) 24 ₁ is located at the peripheryof cell 1 26 ₁ and another WTRU 24 ₂ is located near that WTRU 24 ₁ atthe periphery of another cell, cell 2 26 ₂. The WTRU 24 ₁ of cell 1 26 ₁transmits an uplink communication U1 to its base station 20 ₁ and theWTRU 24 ₂ of cell 2 26 ₂ is receiving a downlink communication, D2, fromits base station 20 ₂. If the uplink communication U1 and the downlinkcommunication D2 are sent in the same spectrum and at the same time, theuplink communication U1 interferes with the downlink communication D2'sreception. Typically, to overcome the interference in its downlinkcommunications, the WTRU 24 ₂ will request an increase in transmissionpower from its base station 20 ₂. The increase in transmission powerresults in increased interference to other WTRUs in and outside its cell26 ₂.

An alternative method of link adaptation is adaptive modulation andcoding (AM&C), in which coding and modulation are adjusted to reduce theinformation data rate in the presence of inter-cellular interference.AM&C decreases the data throughput in the affected WTRU.

In many wireless communication systems, techniques for reducinginterference within a cell are employed. Some of these approachesinclude successive interference cancellers (SICs), parallel interferencecancellers (PICs) and multi-user detectors (MUDs). Although thesetechniques are effective at canceling the intra-cell interference, theytreat inter-cell interference as noise.

Accordingly, it is desirable to reduce inter-cell interference.

SUMMARY

With respect to a first wireless transmit/receive unit (WTRU), at leastone WTRU near the periphery of other cells is determined. Out of the atleast one WTRU near the periphery, at least one WTRU nearby that WTRU isdetermined. The first WTRU codes are provided of the at least one nearbyWTRU. A contribution of the at least one nearby WTRU is canceled from areceived signal at a first WTRU, producing an interference canceledsignal. Data of the first WTRU is detected from the interferencecanceled signal.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is an illustration of inter-cellular interference.

FIG. 2 is a flow chart of inter-cellular interference cancellation.

FIG. 3 is a simplified diagram of an embodiment of an inter-cellularinterference canceller.

FIGS. 4 and 5 are embodiments of the inter-cellular interferencecancellation receiving circuitry.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Although the preferred embodiments are described in conjunction with acode division multiple access communication system, the embodiments areapplicable to other wireless communication systems where there is anoverlap in the used spectrum between cells. Hereafter, a WTRU 24includes but is not limited to a user equipment, mobile station, fixedor mobile subscriber unit, pager, or any other type of device capable ofoperating in a wireless environment. When referred to hereafter, a basestation 20 includes but is not limited to a base station, Node-B, sitecontroller, access point or other interfacing device in a wirelessenvironment.

FIG. 2 is a flow chart for inter-cellular interference cancellation andFIG. 3 is a simplified diagram of an embodiment of such a system.Typically, inter-cellular interference cancellation is performed when aWTRU 24 is experiencing high levels of interference, althoughinter-cellular interference can be employed at lower interference levelsas well. One implementation may be in a slotted code division multipleaccess communication system. In a particular time slot, a WTRU 24 mayexperience high interference levels and in others it may experience lowinterference levels. In such a scenario, the high interference in oneslot may result from a nearby WTRU 24 in another cell transmittingduring that slot.

Initially, WTRUs located at the periphery of cells are determined, step30. One technique for determining the WTRUs at the cell periphery is bymonitoring the power level of the WTRU connections. The WTRUs with thehighest transmission power levels are most likely at the periphery ofthe cell. Also, the pathloss for each WTRU may be used. WTRUsexperiencing more losses are more likely to be at the periphery of thecell. Additionally, for WTRUs having geolocation capabilities, such ascellular based or global positioning based systems the locationinformation along with a map of the cells is used to determine WTRUs atthe cell periphery.

For a WTRU 24 at the cell periphery, a nearby WTRU or nearby WTRUs inneighboring cells are determined, step 32. One approach to determiningnearby WTRUs uses geographic information. For systems using geolocation,WTRUs in close proximity can be determined by comparing their relativepositions. In other systems, the sector that each WTRU resides in may beused to determine nearby WTRUs. To illustrate, WTRUs identified to be atthe periphery of abutting sectors may interfere with each other.

Another approach uses interference measurements. If a WTRU 24 isexperiencing high interference levels in a certain frequency orcombination assigned frequency/time slot, WTRUs transmitting in thatassigned frequency or assigned frequency/time slot are identified.Additionally, received signal strength measurements, such as receivedsignal code power (RSCP), can be used to identify nearby cells. In oneembodiment, only WTRUs from the cell having the highest RSCP measurementare considered, although in other embodiments a group of cells havinghigh RSCP values may be used. Additionally, a combination of theseapproaches may be used to determine nearby WTRUs.

After identifying the interfering WTRU or WTRUs, the codes used by thoseWTRUs are listed, step 34. For implementation with a proposed thirdgeneration partnership project (3GPP) wideband code division multipleaccess (W-CDMA) communication system, the listed codes would includeboth a scrambling and channelization code. The listed codes are signaledto the WTRU. Using these codes, the WTRU cancels the nearby WTRU signalfrom its received signals, step 36. Using the interference cancelledsignal, the WTRU performs a data detection to recover its data, step 38.

By canceling interference from nearby WTRUs, the downlink transmissionpower level to that WTRU 24 can be kept at a lower power level. As aresult, interference to other WTRUs is decreased, increasing the overallcapacity of the system.

In FIG. 3, the base station 20/wireless network 28 receives andtransmits signals over the air interface 54. A measurement collectiondevice 48 collects measurements taken by the base station 20 and WTRUs24, such as pathloss, received signal power measurements andinterference measurements. The measurements are sent to a radio resourcemanagement device 40. An interfering WTRU(s) determining device 44 usesthe collected measurements to determine nearby WTRU(s). An interferingWTRU(s) code listing device 46, lists the codes of the nearby WTRUs. AWTRU(s) code signaling device 50 signals these codes to the WTRU 56. Adownlink transmitter 51 transmits downlink signals to the WTRU 24 usingthe antenna 52.

An antenna 56 at the WTRU 24 receives the downlink signals, theinterfering WTRU codes and interfering signals. A sampling device 58samples the received signals producing a received vector, r. A signalreceiver 62 recovers the interfering codes, C_(I). An interferingWTRU(s) canceller cancels the contribution of the interfering WTRU(s)from the received vector, r, producing r′. Using the interferencecancelled vector, r′, a data detector 64 recovers the data, d, for thatWTRU 24.

FIGS. 4 and 5 are two embodiments of interference canceling receivingcircuits at the WTRU 24. In FIG. 4, signals are received by an antenna66 and a sampling device 68 samples the received signals producing areceived vector, r. A root mean square (RMS) value of the energy of thereceived signals is measured by an RMS measuring device 74. A code ofthe interfering WTRU(s), produced by an interfering WTRU code generator70, is weighted by a weighting device 78 by a value derived from the RMSvalue, analogous to a Widrow filter. Weights could also be estimatedbased on known locations. The weighted WTRU code is subtracted by asubtractor 72 from the received vector r, producing a vector, r′, havingthe contribution of the interfering WTRU removed. In the case of Widrowadaptive noise cancellation, the weight is adaptively selected thatminimizes the RMS output r′ from the RMS measuring device 74. A datadetection using a data detector 76, such as a matched filter, SIC, PICor MUD, is performed on the interference cancelled vector, r′, producinga data vector, d.

In FIG. 5, signals are received and sampled by an antenna 80 and asampling device 82, producing a received vector r. Using the code(s),C_(I), of the interfering WTRU(s), a joint detection is performed by ajoint detector 84 to produce data of the interfering WTRU(s), d_(I). Aninterference reconstruction device 86 reconstructs the contribution,r_(I), of that/those WTRU(s) to the received vector, r. The contributionr_(I) is subtracted from the received vector, r, by a subtractor 88,producing an interference canceled vector, r′. A joint detector 90detects the WTRU data, d.

1. A method for use with a first wireless transmit/receive unit (WTRU)located near a periphery of a cell in a wireless communication system,the method comprising: determining at least one WTRU near the peripheryof other cells; out of the at least one WTRU near the periphery,determining at least one WTRU nearby that WTRU; providing the first WTRUcodes of the at least one nearby WTRU; and canceling a contribution ofthe at least one nearby WTRU from a received signal, producing aninterference canceled signal; and detecting data of the first WTRU fromthe interference canceled signal.
 2. The method of claim 1 wherein thedetermining the at least one WTRU near the periphery of other cellsincludes determining WTRUs of the other cells having a highesttransmission power level to determine the at least one WTRU near theperiphery of the other cells.
 3. The method of claim 1 wherein thedetermining the at least one WTRU near the periphery of other cellsincludes determining WTRUs of the other cells having a highest pathlossto determine the at least one WTRU near the periphery of the othercells.
 4. The method of claim 1 wherein the determining the at least oneWTRU near the periphery of other cells includes determining locations ofWTRUs of the other cells and using a map of the other cells to determinethe at least one WTRU near the periphery of the other cells.
 5. Themethod of claim 1 wherein the determining the nearby WTRU comprisescomparing a location of other cells WTRUs to a location of the firstWTRU.
 6. The method of claim 1 wherein the determining the nearby WTRUcomprises determining out of the at least one near the periphery WTRU atleast one WTRU in a sector abutting a sector of the first WTRU.
 7. Themethod of claim 1 wherein the determining the nearby WTRU comprisesdetermining WTRUs transmitting in a frequency having a large measuredinterference.
 8. The method of claim 1 wherein the determining thenearby WTRU comprises determining WTRUs transmitting in a time slothaving a large measured interference.
 9. The method of claim 1 whereinonly WTRUs in a cell having a highest received signal strength areconsidered as being the at least one nearby WTRU.
 10. A first wirelesstransmit/receive unit (WTRU) capable of canceling interference from asecond WTRU in a neighboring cell, the first WTRU comprising: an antennafor receiving signals including a first signal of the first WTRU, asecond signal of the second WTRU and a third signal indicating a code ofthe second signal; a signal receiver for determining the second signalcode using the received third signal; an interference canceller forcanceling the second signal from the received signals, producing aninterference canceled signal; and a data detection device having aninput configured to receive the interference canceled signal fordetecting data of the first signal.
 11. The first WTRU of claim 10wherein the interference canceller includes: a second code generationdevice for producing a code of the second signal; a weighting device,coupled to a root mean squares (RMS) measuring device, for weighting theproduced code; a subtractor for subtracting the weighted produced codefrom the received signals, producing a subtracted signal; the RMSmeasuring device for measuring a RMS of the subtracted signal; and adata detector having an input configured to receive the subtractedsignal for detecting data of the first signal.
 12. The first WTRU ofclaim 10 wherein the interference canceller includes; a second WTRUjoint detector for detecting data of the second signal using the secondcode; an interference reconstruction device for producing a contributionof the second signal to the received signals; a subtractor forsubtracting the second signal contribution from the received signals,producing a subtracted signal; and a joint detector having an inputconfigured to receive the subtracted signal and for detecting data ofthe first signal.
 13. A first wireless transmit/receive unit (WTRU)capable of canceling interference from a second WTRU in a neighboringcell, the first WTRU comprising: means for receiving signals including afirst signal of the first WTRU, a second signal of the second WTRU and athird signal indicating a code of the second signal; means fordetermining the second signal code using the received third signal;means for canceling the second signal from the received signals,producing an interference canceled signal; and data detection meanshaving an input configured to receive the interference canceled signalfor detecting data of the first signal.
 14. The first WTRU of claim 10wherein the means for canceling includes: a second code generationdevice for producing a code of the second signal; a weighting device,coupled to a root mean squares (RMS) measuring device, for weighting theproduced code; a subtractor for subtracting the weighted produced codefrom the received signals, producing a subtracted signal; the RMSmeasuring device for measuring a RMS of the subtracted signal; and adata detector having an input configured to receive the subtractedsignal for detecting data of the first signal.
 15. The first WTRU ofclaim 10 wherein the means for canceling includes; a second WTRU jointdetector for detecting data of the second signal using the second code;an interference reconstruction device for producing a contribution ofthe second signal to the received signals; a subtractor for subtractingthe second signal contribution from the received signals, producing asubtracted signal; and a joint detector having an input configured toreceive the subtracted signal and for detecting data of the firstsignal.